1. Field of the Invention
The present invention relates to a novel process for producing a high impact resistant thermoplastic graft copolymer composition. This graft copolymer can also be blended with a vinyl chloride polymer to thereby provide a composition having a high impact resistance.
2. Description of the Prior Art
It is well known that rigid and brittle thermoplastic resins such as, for example, polystyrene, polymethyl methacrylate, styrene-acrylonitrile copolymers, polyvinyl chloride or the like can be incorporated with a rubber component to obtain thermoplastic resin compositions having high impact resistance. Such thermoplastic resin compositions include ABS resins, high impact polystyrene resins and the like.
It is possible to obtain copolymer compositions having high impact resistance by polymerizing styrene, acrylonitrile, methyl methacrylate or the like in the presence of a rubber latex prepared by emulsion polymerization. However, it is well recognized that the impact resistance and processing properties of the resultant compositions greatly depend on the particle size of the starting rubber latex, and that the impact resistance and the processing properties are improved as the rubber particle size increases. Therefore, rubber components having large-sized particles are preferably employed in the preparation of high impact resistant resins such as ABS resins or the like. However, the particle size of the rubber latex prepared by the conventional emulsion polymerization techniques is generally in the range of approximately 0.04 to approximately 0.15 micron. Accordingly, various methods for agglomerating rubber particles present in synthetic rubber latexes have been heretofore proposed. The term "particle size" as used in this specification means average diameter of the particles.
The methods for agglomerating rubber particles are classified into two main types; one in which rubber particles are agglomerated during the polymerization step and the other in which rubber particles are agglomerated by treating the small-sized rubber particles after the polymerization step. The most serious defect of the type of method in which rubber particles are agglomerated during the polymeriation step is that an extremely long polymerization time is required to complete the polymerization. For instance, in order to obtain a rubber latex containing rubber particles having a particle size of approximately 0.3 micron, it is necessary to continue the polymerization reaction for 48 to 100 hours. In addition, in this method the largest rubber particle size obtainable is at most approximately 0.4 micron and large amounts of macro-agglomerates or coagulum are generally formed with continued polymerization. Further, in the case where the rubber latex having large amounts of coagulum is used for preparing the desired resin composition, large amounts of coagulum are also formed in the graft polymerization step.
On the other hand, the other of method in which rubber particles are agglomerated by treating small-sized rubber particles after the polymerization step can agglomerate the small-sized rubber particles in a relatively short time. Examples of this method are: the treatment of a rubber latex with an acid, a metallic salt, an ammoniumsoap, a particular solvent or a high molecular colloidally active chemical; a freezing and thawing technique, and; a method for treating a rubber latex under high pressure or the like. However, this type of method is disadvantageous in that special apparatuses or additives are required to agglomerate rubber latexes and to produce high impact resistant resins. In addition, in the case wherein the rubber latex is agglomerated by addition of an acid or a metallic salt, special apparatus is not required but the agglomerated particle size of the latex is greatly influenced by the concentration of the acid or the salt to be added, the rate of its addition and the agitating condition of the latex system. This is because when an acid or a slat is used, the particles which are agglomerated are naturally agglomerated by partial destruction of the rubber latex emulsion. Accordingly, it is generally impossible in this method to agglomerate the rubber particles to a size of more than 0.3 micron without the formation of macro-agglomerates or coagulum. In addition, an acid or a salt is used in the form of an aqueous solution of a low concentration, so that the solid concentration of the rubber latex is considerably lowered and the production rate is decreased.